1. Field of the Invention
The present invention relates to the field of fabrication of integrated circuits, and, more particularly, to a technique for mechanically aligning a substrate with respect to a process tool, such as a photolithography tool.
2. Description of the Related Art
Fabrication of integrated circuits requires the precise formation of very small features with a very small tolerance for error. Such features may be formed in a material layer formed above an appropriate substrate, such as a silicon substrate. These features of precisely controlled size are generated by patterning the material layer by performing known photo-lithography and etching processes, wherein a masking layer is formed over the material layer to be treated to define these features. Generally, a masking layer may consist of or is formed by means of a layer of photoresist that is patterned by a lithographic process. During the lithographic process, the resist may be spin-coated onto the wafer surface and is then selectively exposed to ultraviolet radiation. After developing the photoresist, depending on the type of resist, positive resist or negative resist, the exposed portions or the non-exposed portions are removed to form the required pattern in the layer of photoresist. Since the dimensions of the patterns in sophisticated integrated circuits are steadily decreasing, the equipment used for patterning device features have to meet very stringent requirements with regard to resolution and overlay accuracy of the involved fabrication processes. In this respect, resolution is considered as a measure specifying the consistent ability to print images of a minimum size under conditions of predefined manufacturing variations. One important factor in improving the resolution is represented by the lithographic process, in which patterns contained in a photo mask or reticle are optically transferred to the layer of photo-resist via an optical imaging system. Therefore, great efforts are made to steadily improve optical properties of the lithographic system, such as numerical aperture, depth of focus and wavelength of the light source used. The quality of the lithographic imagery is extremely important in creating very small feature sizes.
Of at least comparable importance, however, is the accuracy with which an image can be positioned on the surface of the substrate. Integrated circuits are typically fabricated by sequentially patterning material layers, wherein features on successive material layers bear a spatial relationship to one another. Each pattern formed in a subsequent material layer has to be aligned to a corresponding pattern formed in the previously patterned material layer within specified registration tolerances.
These registration tolerances are caused by, for example, a variation of a photoresist image on the substrate due to non-uniformities in such parameters as resist thickness, baking temperature, exposure and development. Furthermore, non-uniformities of the etching processes can also lead to variations of the etched features. In addition, there exists an uncertainty in overlaying the image of the pattern for the current material layer to the pattern of the previously formed material layer while photolithographically transferring the image onto the substrate. Several factors determine the ability of the imagery system to overlay two layers, i.e., the existing layer and the layer to be transferred from the reticle to the substrate, such as imperfections within a set of masks, temperature differences at the different times of exposure, and a limited alignment capability of the alignment tool, which is commonly a part of the photolithography tool.
In commonly available photolithography tools, such as steppers that accomplish exposure of substrates in a step-and-repeat process, the substrates are typically aligned in a two-step procedure, wherein first in a so-called pre-alignment, a coarse orientation of the substrate is achieved in that prominent positions of the substrate, located, for example, at the substrate edge, are adjusted such that alignment marks within the substrate are positioned within a specified capture range of a fine alignment system. Then, in a subsequent fine alignment, the actual registration of the substrate, or portions thereof when a die-by-die: alignment is required, is accomplished. During the two-step alignment, the substrate is placed on a substrate stage and is then aligned by, for example, a two-dimensional translation and a rotation in the plane defined by the two-dimensional translations, with respect to tool specific alignment marks. The accuracy of the alignment depends on, among other things, how the incoming substrate is placed on the substrate stage. This process may provide appropriate precision when the pre-alignment process is able to position the substrate with a sufficient degree of precision that allows the subsequent fine alignment routines to obtain the finally required accuracy. Thus, when the pre-alignment leads to an alignment result not falling within a specified process “window,” a process abort may result, since the fine alignment procedure may not be able to locate alignment marks on the substrate, thereby significantly reducing the throughput of the lithography tool. In other cases, an inappropriate pre-alignment may entail a significant alignment error owing to a certain “periodicity” of the fine alignment procedure, yet resulting in a precise orientation, however, with a considerable translatory offset corresponding to the periodicity. Consequently, reworking of the substrate is required when the alignment error is detected by inspection, or a failure in subsequent processes may occur when the misalignment remains undetected. In either case, the throughput of the lithography tool is considerably reduced.
The pre-alignment process may be based on tool specific constants, such as offset values for the tool focus, basic settings for sensor and actuator elements, and the like. These constants may be determined during a qualification process, thereby possibly taking into account a specified type of substrates having experienced a specified one or more process sequences. However, a parameter drift in the tool and/or variations on the substrate side may lead to significant variations of the alignment procedure and thus entail the above-discussed disadvantages.
In view of this situation, it is desirable to provide a technique for aligning substrates on the basis of alignment tool specific and substrate specific parameters, wherein a parameter drift may be compensated for or at least be considerably reduced.